NMDA receptors play critical roles in the regulation of synaptic plasticity, neuronal development and several neurological and psychiatric diseases. Recent studies have shown that NMDA receptors bind to the PSD95/SAP9O adaptor protein to form a large macromolecular signaling complex. The PSD95/SAP9O protein family appears to play a critical role in the synaptic targeting of NMDA receptors and in the coupling of NMDA receptors to downstream signal transduction pathways. In this research proposal we plan to study the structure, function and regulation of the NMDA receptor macromolecular signaling complex and the role of this complex in synaptic transmission and plasticity. Recent studies in our lab have shown that phosphorylation of the NMDA receptor by a novel protein kinase disrupts receptor binding to the PSD95/SAP9O protein and thus may play a critical role in the regulation of NMDA receptor signaling. In the proposed research we plan to characterize the kinase responsible for this phosphorylation and examine the effect of this phosphorylation on NMDA receptor synaptic targeting and downstream signaling. Our laboratory has also recently shown that PSD95/SAP9O binds to a novel synapse specific rasGAP, SynGAP, which may regulate ras signaling at excitatory synapses. In this proposal we plan to analyze the role of SynGAP in the regulation of synaptic ras signaling and synaptic transmission and plasticity. The function of SynGAP will be studied in neurons transfected with wild-type and mutant forms of SynGAP and in recently obtained SynGAP knock-out mice. Finally, our laboratory has also recently found that the Rsk2 protein kinase binds to the synaptic scaffolding protein SHANK. SHANK is a component of the NMDA receptor complex that also interacts with several other synaptic proteins including GKAP, Homer and metabotropic glutamate receptors. Interestingly Rsk2 phosphorylates SHANK and may regulate the functional properties of the NMDA receptor complex. In this proposal we will further characterize the phosphorylation of SHANK by Rsk and analyze the functional effects of this phosphorylation on the regulation of synaptic function and plasticity. These studies investigate three different levels of the NMDA receptor protein complex and will help elucidate the function of this large macromolecular complex in excitatory synaptic function and its potential role in neurological and psychiatric diseases.